As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, a multi function peripheral thereof and the like, there has been known an inkjet recording apparatus and the like employing a liquid discharging recording method using a recording head discharging ink droplets or the like. In the image forming apparatus employing the liquid discharging recording method, an image is formed by discharging ink droplets from a recording head onto a fed sheet. Herein, the term “forming” is a synonym of the terms recording, typing, imaging, and printing. Further, herein, the term “sheet” is not limited to paper, and refers to any appropriate medium (e.g., the OHP) to which ink droplets, other liquid and the like may be adhered. The image forming apparatus employing the liquid discharging recording method may be classified into two types: a serial-type image forming apparatus and a line-type image forming apparatus. In the serial-type image forming apparatus, an image is formed by discharging ink droplets from the recording head while the recording head moves in the main scanning direction. On the other hand, in the line-type image forming apparatus, an image is formed by discharging ink droplets from the line-type recording head while the recording head does not change its position.
Further, in an embodiment of the present invention, the term “image forming apparatus” employing the liquid discharging recording method refers to an apparatus forming an image by discharging a liquid onto a medium including paper, thread, fiber, textile, leather, metal, plastic, glass, wood, ceramics and the like. Further, the term “image forming” refers to not only forming a meaningful image such as characters, figures, and the like on a medium but also forming a meaningless image such as a pattern and the like on a medium (including simply discharging droplets onto a medium). Further, the term “ink” is collectively used and herein refers to not only any material called “ink” but also any liquid for forming an image which may be called recording liquid, fixing processing liquid, liquid, a DNA sample, a patterning material, a resin and the like. Further, the “image” is not limited to a planar image. For example, the “image” includes an image formed on a material that is three-dimensionally formed and an image three-dimensionally formed made of three-dimensional figures.
Among such image forming apparatuses there is a known image forming apparatus employing an ink supply method in which ink is supplied from a main tank to a sub tank, the main tank (a.k.a. an ink cartridge) being detachably mounted on a main body of the apparatus, the sub tank (also called a head tank or a buffer tank) supplying ink to the recording head.
Among such image forming apparatuses there is a known image forming apparatus having the sub tank (also called a head tank or a buffer tank) supplying ink to the recording head, the sub tank having a negative pressure forming function (mechanism) for generating a negative pressure to prevent a leakage (ooze) or drop of ink from the recording head. The sub tank includes a negative pressure forming unit including a flexible member (film member) forming one surface of the ink container containing ink and an elastic member biasing the flexible member toward outside. The sub tank further includes an air opening mechanism for opening (releasing) the inside of the ink container to the atmosphere, so that ink can be supplied from the ink container to the recording head.
Further, the sub tank further includes a displacement member (which may also be called a detection member or a detection filler) that is displaced in accordance with the displacement of the flexible member. During air opening filling where the sub tank is opened to the atmosphere using the air opening mechanism and ink is supplied from the main tank to the sub tank, the carriage is moved to a predetermined detecting position (full tank filling position). To that end, after the sub tank is air-opened by driving a driving unit provided on the main body side for driving the air opening mechanism, the carriage is moved to the predetermined position, and the ink supply is performed. When the detecting unit on the main body side detects the displacement member, the full tank filling position is determined (Patent Documents 1 through 9).
In this case, to make it possible to supply ink during printing, when ink consumption during printing is equal to or greater than a first predetermined value, based on the information in correlation with the ink supply amount supplied from the main tank to the sub tank during the printing, if the ink supply amount is equal to or less than a second predetermined value, ink is supplied from the main tank to the sub tank, and if the ink supply amount is greater than a second predetermined value, ink is not supplied from the main tank to the sub tank (Patent Document 9).
In another example, the sub tank does not have the structure as described above. Instead, the sub tank includes remaining ink amount detection unit so as to supply ink even during printing (Patent Document 10).    [Patent Document 1] Japanese Patent No. 4298474    [Patent Document 2] Japanese Patent No. 4190001    [Patent Document 3] Japanese Patent No. 4155879    [Patent Document 4] Japanese Laid-Open Patent Application No. 2007-015153    [Patent Document 5] Japanese Laid-Open Patent Application No. 2007-130979    [Patent Document 6] Japanese Laid-Open Patent Application No. 2008-132638    [Patent Document 7] Japanese Laid-Open Patent Application No. 2009-023329    [Patent Document 8] Japanese Laid-Open Patent Application No. 2009-274325    [Patent Document 9] Japanese Laid-Open Patent Application No. 2009-023092    [Patent Document 10] Japanese Patent No. 3219326